Apparatuses such as printers and copying machines that print or form color images are making great progresses in terms of both performance and penetration. In addition, to print or form full-color images, many schemes including silver halide scheme, thermal printing, electrophotography, electrostatic printing, and inkjet printing have been developed, and high-quality full-color images can be obtained. However, a new problem rises under these circumstances.
That is, bank notes and securities can easily be forged using a full-color image printing apparatus. For this reason, full-color image printing apparatuses must have a function of preventing forgery. Recent full-color image printing apparatuses have various forgery preventing functions. The most general scheme of implementing a forgery preventing function is so-called pattern tracking in which in printing an image, a periodic dot pattern representing the machine number of the apparatus or the like is printed on the printing paper sheet. If a forged bank note or the like is found, the machine number is detected from the dot pattern to specify the apparatus used for forgery. The dot pattern is printed on all printing paper sheets to be output from the apparatus and is therefore generally printed using the most unnoticeable color, i.e., yellow.
In an apparatus for adding a different kind of information except an image by printing such a dot pattern on a printing paper sheet together with an image (to be expressed as “embedding a dot pattern in an image” hereinafter), the influence of the embedded dot pattern on the image and the accuracy of the different kind of information detected from the printed image depend on the shape of an on-dot pattern (to be described later) as a component of the embedded dot pattern. That is, use of an on-dot pattern that mainly aims at suppressing any degradation in image quality sacrifices accurate information detection. Conversely, use of an on-dot pattern that aims at improving the detection accuracy largely degrades the image quality.